Hi Philipp,
I try to do exactly what you do, but I still have problems … My divi version is 3.17.6, and I think you have an older version, because the sub panel ‘link’ in ‘blurb’ module is different form my version.
In fact I have 4 voices: two for the title url and two for the module url. So I think that something is changed in divi modules since you developed your plugin.
Now I use the call-to-action module to do the same thing (rkind of the same thing).
Massimo